docs: def Robbins(L, G, rhol, rhog, mul, H=1.0, Fpd=24.0):
      r'''Calculates pressure drop across a packed column, using the Robbins
      equation.
  
      Pressure drop is given by:
  
      .. math::
          \Delta P = C_3 G_f^2 10^{C_4L_f}+0.4[L_f/20000]^{0.1}[C_3G_f^210^{C_4L_f}]^4
  
      .. math::
          G_f=G[0.075/\rho_g]^{0.5}[F_{pd}/20]^{0.5}=986F_s[F_{pd}/20]^{0.5}
  
      .. math::
          L_f=L[62.4/\rho_L][F_{pd}/20]^{0.5}\mu^{0.1}
  
      .. math::
          F_s=V_s\rho_g^{0.5}
  
      Parameters
      ----------
      L : float
          Specific liquid mass flow rate [kg/s/m^2]
      G : float
          Specific gas mass flow rate [kg/s/m^2]
      rhol : float
          Density of liquid [kg/m^3]
      rhog : float
          Density of gas [kg/m^3]
      mul : float
          Viscosity of liquid [Pa*s]
      H : float
          Height of packing [m]
      Fpd : float
          Robbins packing factor (tabulated for packings) [1/ft]
  
      Returns
      -------
      dP : float
          Pressure drop across packing [Pa]
  
      Notes
      -----
      Perry's displayed equation has a typo in a superscript.
      This model is based on the example in Perry's.
  
      Examples
      --------
      >>> Robbins(L=12.2, G=2.03, rhol=1000., rhog=1.1853, mul=0.001, H=2.0, Fpd=24.0)
      619.6624593438102
  
      References
      ----------
      .. [1] Robbins [Chem. Eng. Progr., p. 87 (May 1991) Improved Pressure Drop
         Prediction with a New Correlation.
      '''
      # Convert SI units to imperial for use in correlation
      L = L*737.33812 # kg/s/m^2 to lb/hr/ft^2
      G = G*737.33812 # kg/s/m^2 to lb/hr/ft^2
      rhol = rhol*0.062427961 # kg/m^3 to lb/ft^3
      rhog = rhog*0.062427961 # kg/m^3 to lb/ft^3
      mul = mul*1000.0 # Pa*s to cP
  
      C3 = 7.4E-8
      C4 = 2.7E-5
      Fpd_root_term = (.05*Fpd)**0.5
      Lf = L*(62.4/rhol)*Fpd_root_term*mul**0.1
      Gf = G*(0.075/rhog)**0.5*Fpd_root_term
      Gf2 = Gf*Gf
      C4LF_10_GF2_C3 = C3*Gf2*10.0**(C4*Lf)
      C4LF_10_GF2_C3_2 = C4LF_10_GF2_C3*C4LF_10_GF2_C3
      dP = C4LF_10_GF2_C3 + 0.4*(5e-5*Lf)**0.1*(C4LF_10_GF2_C3_2*C4LF_10_GF2_C3_2)
      return dP*817.22083*H